In 1979, forgotten Alaska salmon cans revealed a rising worm count, exposing an unexpected ocean recovery

For decades, a store of musty salmon cans sat overlooked in a warehouse, only to be tapped by scientists as the source of surprising ecological findings in the North Pacific.Researchers looking through old cans of Alaskan salmon found that the incidence of parasites, known as anisakids, became significantly more common in some salmon species between 1979 and 2021. While normally we'd see an increase in parasites as a bad thing, scientists say the increase could be a sign of a more complex and recovering marine food web.According to the study published in the journal Ecology and Evolution, available through PubMed, researchers used a 40-year archive of canned salmon to reconstruct long-term trends in parasite abundance that would otherwise have been difficult to study. Researchers found that using preserved canned fish would provide researchers a unique, historical snapshot of parasite burden that may otherwise not have been accessible. They analysed four commercially important salmon species based on the abundance of parasites over 40 years using archived canned fish as a rare "window into the past.”Opening decades-old cansThis archive of canned fish provided an invaluable, yet unexpected, resource for researchers aiming to understand long-term changes in the distribution and prevalence of parasites in marine environments. These parasites depend on several species within a complex marine food web, so an increase in parasites might reflect a rise in the marine mammals or other species they require to complete their life cycle.The researchers examined 178 cans of Alaskan salmon processed between 1979 and 2021, including chum, coho, pink, and sockeye salmon. The fish were preserved by the Seafood Products Association to study product quality and shelf life. In total, they recovered 372 worms from the archived samples.The researchers found that parasite burden had increased in pink and chum salmon over time, but did not show this trend in sockeye or coho.Why might more parasites be a good sign?The parasites belong to the family Anisakidae, a group of worms that rely on a complex marine life cycle.The larvae are ingested by tiny marine organisms, passed on to fish, like salmon, in whose flesh they embed, and eventually consumed by marine mammals that host adult worms. An increase in parasite numbers is often a reflection of an increase in other species involved in the parasite's life cycle. The authors note that some marine mammal populations have recovered following protections introduced under the Marine Mammal Protection Act in 1972. Climate change may also play a roleThe researchers also noted that the relationship between increasing parasite loads and increasing marine mammal populations is seen in other parts of the world, like the Baltic Sea. It's possible that there are multiple factors involved, though, as the researchers suggest higher temperatures caused by global warming may also lead to faster worm growth and the expanded geographic range of hosts."Climate change may also contribute by speeding up anisakid growth and reproduction," the authors wrote in their conclusion.What does this mean for consumers?For most people, the findings are not a food safety concern.Live anisakids can cause illness when eaten raw in sushi or undercooked, but commercial canning processes and even cooking kill the parasites, removing the infection risk. Allergic reactions can occur, even to dead parasites, but this is rare.One of the most remarkable findings from this study may be the nature of the archive itself. It's extremely difficult to get long-term data on parasites and most other marine ecological systems. Historical collections and parasite monitoring programs have limitations, with little historical data to draw on.The canned salmon provided a biological time capsule that allowed researchers to track changes in parasite abundance over decades. The study highlights how archival collections can provide unexpected insights into long-term ecological change.The authors hope to bring attention to the importance of parasite monitoring for conservation efforts and public health.